The nebular Heii\(\lambda \) 4686 emission observed in some SF
galaxies has often been attributed to X-ray emission from HMXBs
[1]}, [2]}. This hypothesis has been critically
tested by [3]} using photoionization grids
accounting for the combined contribution of HMXBs and young massive
stars for metallicities from Z=0.02 to Z=0.001 (\(\simeq \) 5\(\%\) 
solar), and provide predictions for the Heii/H\(\beta \)  ratio
that correspond to a range of values of the \(L_{\rm X,0.5-8~keV}\) /SFR
ratio. We estimate a SFR of 0.14 M\(_\odot \)  yr\(^{-1}\)  for IZw18The IZw18 integrated \(F(H\alpha )\) =4.36\(\times \) 10\(^{-13}\) erg s\(^{-1}\)  cm\(^{-2}\)  (K16) was translated to \(L(H\alpha )\)  assuming a distance of 18.2 Mpc.
based on its \(L(H\alpha )\) =1.73\(\times \) 10\(^{40}\) erg s\(^{-1}\) (K16) and
the widely used SFR-L(H\(\alpha \) ) relation from [4]}. Therefore, taking the
\(L_{\rm X,0.5-8~keV}\)  value in Table REF  derived from
the 2002-04-10-XMM-Newton data set, this implies an observed
\(L_{\rm X,0.5-8~keV}\) /SFR of 9\(\times \) 10\(^{40}\) 
erg s\(^{-1}\) /(M\(_\odot \)  yr\(^{-1}\) ). According to S20's
predictions, this \(L_{\rm X,0.5-8~keV}\) /SFR value results in Heii/H\(\beta \)  between 0.0004 to 0.0025, regardless of the metallicity
assumed in the models, a value well below the observed Heii/H\(\beta \)  of
0.023\(\pm \) 0.004 in IZw18 (K16).
To reinforce this result, we have computed the total Heii-ionizing photon
flux \(Q\) (Heii) from Kerr BH and irradiated disk models of the X-ray
emission detected in the 2002-04-10-XMM-Newton observation.
The thin accretion disk around a Kerr BH model including a
Comptonization component provides an extrapolation to UV wavelengths
\(<\)  228 Å similar to that of the multicolor disk models
used by S20.
The best fit for this model has very similar parameters to those reported
by [5]}, with an acceptable fitting quality (\(\chi ^2\) /DoF=0.90).
The intrinsic (unabsorbed) \(\log Q\) (Heii) for this model is
\(\approx \) 48.4 photon s\(^{-1}\) , i.e.,\(\approx \) 50 times lower than
that of \(\log Q\) (Heii)=50.1 photon s\(^{-1}\)  reported by K15.
Meanwhile, the emission model of a disk irradiated by the Compton tail
[6]} is of particular interest, as the physical processes of
disk irradiation provide a meaningful description of the expected UV
spectrum to constrain more reliably \(Q\) (Heii) [7]}.
The best fit for this model has a slightly better fit quality
(\(\chi ^2\) /DoF=0.93) with a local \(N_{\rm H}\) =1.5\(\pm \) 0.8\(\times \) 10\(^{21}\)  cm\(^{-2}\) ,
inner disk temperature 0.55\(\pm \) 0.25 keV,
photon index 2.9\(\pm \) 0.6, and irradiated flux fraction of 2.3\(\%\)  for the
[6]}.
The resulting intrinsic (unabsorbed) \(\log Q\) (Heii) is
\(\approx \) 49.1 photon s\(^{-1}\) , which is still 10 times lower than
observed.
It is worthwhile noting that the \(L_{\rm X}\)  derived from
these models is very similar to that derived from a power-law model.
